Field of the Invention
This invention relates to free flowing powders and to an improved method for producing them from finely divided particulate material.
Free flowing powders for flame spraying have been made by various agglomeration methods which make free flowing powders of normally non-flowing small-diameter powder particles. These methods usually involve the use of an organic binder which causes many small particles to stick together resulting in agglomerates of larger size and relatively lower surface area and consequently have improved flow properties.
One of the more sophisticated agglomeration methods used for some time in the pharmaceutical and food industries utilizes spray drying. Agglomerates are formed in spray drying by atomizing a slurry of powder, binder and liquid into a drying chamber where the liquid is evaporated. The result is a generally spherical agglomerate held together by the binder.
Spray drying has been used in the production of flame spray powders. See U.S. Pat. No. 3,617,358, issued Nov. 2, 1971. This patented procedure as well as other methods of agglomeration do produce free flowing powders, however, not without some undesirable characteristics, most of which are related to the presence of the organic binder.
An organic binder has little other beneficial contribution than the ability to hold the particles together. Powder with organic material present does not work well in commercially available flame spray equipment. In many cases the binder is not strong enough to withstand handling and feeding. If strong enough for production use it vaporizes in the flame causing smoke and will condense in cooler areas, causing plugging or fouling of the gun, workpiece or work area.
It has been suggested and tried to remove the binder by various firing conditions. This procedure will result in powder without the organic material but not often without some trace of contamination.
Another difficulty with the binder is that it occupies space which powder could otherwise occupy and in this way holds these powders to relatively low apparent densities.